The A’s had won 19 of 24 and took three of four from AL West-leading Houston before falling to their rivals from across the San Francisco Bay.
Oakland All-Star second baseman Jed Lowrie was forced out of the game with a left leg contusion following a collision with Stephen Piscotty while both chased Alen Hanson’s fly ball to short right field in the third.
Lowrie’s status for Tuesday’s All-Star Game in Washington is uncertain. He was selected to the AL team as an injury replacement for Gleyber Torres of the New York Yankees.
Chad Pinder homered for Oakland.
Neither team did much offensively early and the Giants scored their first run on a balk by A’s starter Edwin Jackson (1-1) in the fourth.
Posey came through in the sixth.
Posey was batting fifth for the first time this season as part of manager Bruce Bochy’s attempt to shake things up.
Bumgarner (3-3) allowed one run with five strikeouts in his third consecutive win over the A’s. The left-hander retired 14 straight during one stretch and left after giving up a single and two walks in the seventh.
Reyes Moronta replaced Bumgarner and retired three straight to get out of the jam.
Jackson yielded two runs over six innings but lost in his second attempt at his 100th career win. Jackson struck out five and walked three.
Athletics: Khris Davis did not start because of a sore left shoulder but played left field after Piscotty moved to second when Lowrie was removed. . RHP Josh Lucas was optioned to Triple-A Nashville. LHP Jeremy Bleich was called up. RHP Andrew Triggs was transferred to the 60-day disabled list.
RHP Jeff Samardzija (1-5, 6.42 ERA) tries to get his injury-riddled season back on track for San Francisco in the middle game of the series. LHP Brett Anderson (1-2 5.45), who was part of a combined five-hit shutout in his last start, goes for Oakland.